We report on the development of a field-proven distributed fiber optic sensing system for structural health monitoring in road construction and civil engineering. The system is based on a cost-efficient digital incoherent optical frequency domain reflectometry (I-OFDR) for distributed strain detection along a polymer optical fiber. In this method, the strain-induced backscatter increase in a graded-index multimode perfluorinated polymer optical fiber (PF-POF) is determined by measuring the complex transfer function of the sensing fiber using a compact digital data acquisition unit. This unit replaced an oversized vector network analyzer (VNA) usually used in the I-OFDR technique making the measurement system more robust and more suitable for its use in the field, whilst at the same time providing a significant reduction of the total sensor system costs. This paper presents a successful implementation of the entire sensor concept in a real construction project for the soil reinforcement and fiber optic monitoring of a road embankment using geosynthetics with incorporated PF-POF. The presented research includes development and installation in the field of the sensors integrated geosynthetics used for ground stabilization and additionally providing fiber optic POF-based monitoring function. The publication shows first measurement results of the further developed measurement method I-OFDR conducted along the PF-POF integrated in the geosynthetics embedded into a road embankment of the federal road B91 south from Leipzig.
We propose a digital implementation of the incoherent optical frequency domain reflectometry (I-OFDR) technique for precise backscatter measurement and optical fiber sensing applications. Specific performance parameters of the I-OFDR are discussed and compared to an analog vector network analyzer-based I-OFDR system. Improved sensitivity, dynamic range and signal stability of the digital I-OFDR is presented and demonstrated by means of quasi-distributed length change measurement.
Technical textiles with embedded distributed fiber optic sensors have been developed for the purposes of structural
health monitoring in geotechnical and civil engineering. The distributed fiber optic sensors are based on Brillouin
scattering in silica optical fibers and OTDR in polymer optical fibers. Such "smart" technical textiles can be used for
reinforcement of geotechnical and masonry structures and the embedded fiber optic sensors can provide information
about the condition of the structures and detect the presence of any damages and destructions in real time. Thus,
structural health monitoring of critical geotechnical and civil infrastructures can be realized. The paper highlights the
results achieved in this innovative field in the framework of several German and European projects.
Terrorists and criminals more and more attack and destroy important infrastructures like routes, railways, bridges,
tunnels, dikes and dams, important buildings. Therefore, reliable on-line and long-term monitoring systems are required
to protect such critical infrastructures. Fiber optic sensors are well-suited for that. They can be installed over many
kilometers and are able to measure continuously distributed strain, pressure, temperature and further mechanical and
physical quantities. The very tiny optical fibers can be integrated into structures and materials and can provide
information about any significant changes or damages of the structures. These so-called smart materials and smart
structures are able to monitor itself or its environment. Particularly smart technical textiles with embedded fiber optic
sensors have become very attractive because of their high importance for the structural health monitoring of geotechnical
and masonry infrastructures. Such textiles are usually used for reinforcement of the structures; the embedded fiber optic
sensors provide information about the condition of the structures and detect the presence of any damages and
destructions in real time. Thus, critical infrastructures can be preventively protected. The paper will introduce this
innovative field and will present the results achieved within several German and European projects.
This article demonstrates how spatial resolution of distributed Brillouin sensing systems can be significantly
enhanced by measuring the spatially resolved Brillouin gain spectra of an optical fiber in the frequency domain.
We employ a novel signal processing method to overcome the known problem of spectral broadening that occurs
at centimeter-range spatial resolutions. The method is based on an analytical description of stimulated Brillouin
scattering in optical fibers for the case of harmonically modulated optical signals. From this analysis, the artifacts
that degrade the measurement resolution were modeled; for the first time to our knowledge, undegraded gain
spectra could be restored by means of deconvolution techniques adopted from digital image processing.
We report on the development of a distributed sensor system for strain measurement using Brillouin optical
frequency domain analysis (BOFDA) in single-mode silica optical fibers. Our research aims at the application of
the sensor system in flood protection. The sensing fibers are embedded into the soil body of river dikes, where
they perform early detection of critical soil displacement. We present a BOFDA setup that performs strain
measurements with a spatial resolution better than 3 meters over a length of 2 km. Its accuracy is verified by
measurements on a calibrated strain profile as well as several laboratory tests that emulate the stressing of the
optical fibers by soil movement. It is shown that the BOFDA approach offers feasible solutions to known critical
issues of Brillouin sensing such as spectral broadening at high spatial resolution, digital filtering for enhancement
of the dynamic range, and fluctuations of the Brillouin gain due to birefringence.
We report on the development of a complete system for spatially resolved detection of critical soil displacement in
river embankments. The system uses Brillouin frequency domain analysis (BOFDA) for distributed measurement
of strain in silica optical fibers. Our development consists of the measurement unit, an adequate coating for the
optical fibers and a technique to integrate the coated optical fibers into geotextiles as they are commonly used in
dike construction. We present several laboratory and field tests that prove the capability of the system to detect
areas of soil displacement as small as 2 meters. These are the first tests of truly distributed strain measurements
on optical fibers embedded into geosynthetics.
KEYWORDS: Optical fibers, Multimode fibers, Single mode fibers, Signal attenuation, Scattering, Phase only filters, Temperature metrology, Structural health monitoring, Silica, Fusion splicing
We investigate the suitability of silica graded index multimode fibers (MMF) for distributed Brillouin sensing
in structural health monitoring, where the measurement range is limited by small bendings that appear during
the integration process of the sensing fibers into the structures. For the investigation of stimulated Brillouin
scattering (SBS) in MMF, we use an MMF connected on both ends to the SMF measurement setup by fusion
splices to ensure that only the fundamental mode is transmitted. The SBS spectra in MMF are recorded using
a 1319 nm single frequency (line width 5 kHz) laser. Results found for standard singlemode fibers and the
fundamental mode in multimode silica optical fibers are compared. We present the gain spectra showing the
dependence of frequency shift, attenuation and modal noise to both temperature and strain. The dependence
of the attenuation due to bending is shown. Finally, the perspective of the excitation of SBS in polymer optical
fibers is discussed against the background of our research on SBS in MMF.
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